As an airplane flies faster than the speed of sound, it "pushes" on the sound waves in front of it. But sound waves obey the speed limit—they can't travel faster than the speed of sound.

The oscillating variations in sound pressure (called the WAVEFORM of the sound) PROPAGATE in the form of a SOUND WAVE. See: ACOUSTIC IMPEDANCE, COMPRESSION, LAW OF SUPERPOSITION, PARTICLE VELOCITY, ...